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Fitting a Supercharger, Part 3

Organising the belt drive

by Julian Edgar

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At a glance...

  • Part 3 of a 4-part series
  • Using existing belt drives
  • Poly-vee belts
  • Tensioners
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This article was first published in 2005.

Most small superchargers available from wreckers use a multi-rib belt drive. Depending on the car to which the bower is being fitted, this drive can be taken from a standard accessory belt. However, it’s all a bit more difficult to do in practice than it sounds when it’s said quickly!

Using an Existing Belt Drive

If the blower can be positioned so that its drive pulley lines up perfectly with an existing belt drive, a longer than standard belt may be able to be used to additionally drive the blower. However, before even considering this approach, a number of factors need to be assessed.

Some cars use a single serpentine belt to drive a bunch of things including the power steering, air con and alternator. In these cars, the belt drive will be wide (eg a 6 rib belt), include a sophisticated tensioner, and be driven from a large crankshaft pulley. Getting hold of a longer belt is likely to be easy and the original tensioner can continue to be used.

However, other cars use separate belts to drive some devices – for example, a single belt to the power steering pump. This belt may be a relatively narrow, its tension may be able to be set only over a small range by adjusting the power steering pump on its mounts, and the crankshaft drive pulley may be small. Using this belt drive for the supercharger may have a host of associated problems.

Key questions that need to be answered when considering the use of an existing belt drive system include:

  • Can the supercharger drive pulley be lined up with an existing belt drive?
  • Does the existing belt width match the supercharger pulley?
  • Does the belt have a tensioning arrangement that’s suitable if a longer belt also drives the blower?
  • Is the crankshaft pulley diameter suitable for driving the blower? If not, can the supercharger drive pulley be changed?
  • Is there clearance for the belt on its new path?
  • Is a longer belt available?
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Multi-rib belts are technically known as ‘poly vee’ or ‘poly grooved’ belts. In automotive use, the cross-sectional profile of the ribs is standardised (it’s a "PK" profile) and only the number of ribs and the length of the belt varies.

A simple code is used – a 6PK 1410 belt is one with 6 ribs and a length of 1410mm. Note that this code number is written on the belt itself, so there’s no need to know the car it’s usually fitted to. This is very useful because it allows you to measure the required new belt length, count the ribs and then go to an auto parts store and ask for a 4PK 1460 – or whatever.

Another very important point to know is that it’s easy to reduce the number of ribs by using a sharp knife to slice the belt longitudinally. So for example, if cut in half, a 6PK belt can become a 3PK belt, or if two of the grooves are cut off, a 6PK can become a 4PK.

In the case of the blower fitting being covered in this series, finding out that belts could be altered in rib count was an absolute saviour – the blower brackets were already made and a 3PK belt around 1460mm long was needed – but no such belt exists. However, there are plenty of 6PK belts around this length – and as a bonus, after it has been slit into two, you have two belts for the price of one!

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The greater the number of ribs, the greater the width of the belt and the more power it can transmit. Note that compared with the older type of narrow V-belts, poly-vee belts need higher belt tension – something to keep in mind if a dedicated tensioner is required. Another characteristic of poly-vee belts is that their rear (ie flat) surface can be used to drive devices. However, this side of the belt cannot transmit as much power as the ribbed side.


Tensioners are available in two basic designs – those that are manually adjusted and those that use an internal spring to automatically maintain belt tension. A tensioner used with a poly-vee belt the can be placed on the inside of the belt (a grooved pulley is used) or on the outside of the belt (a flat pulley is used).

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In addition to allowing the adjustment of belt tension, a tensioner can also perform other functions. Firstly, it can act as an idler pulley. In this role it helps guide the belt when there is a great distance between successive pulleys, and can also alter the path of the belt to provide clearance. Secondly, a tensioner can be used to improve the supercharger belt wrap – that is, the number of degrees that the belt is in contact with the pulley. If it is to improve the wrap, the tensioner will probably need to be placed on the outside of the belt and so in the case of a poly-vee belt, will need to use a flat pulley.

(In actual fact, ‘flat’ tensioner pulleys are not flat but instead have a slightly crowned surface. The belt rides up the crown and so centres itself without shoulders being required on the pulley.)

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In a belt drive system there are parts of the belt which are relatively slack and parts which are relatively taut. Except for some Hondas, all engines rotate clockwise as viewed front-on looking at the crank pulley. Belts work by pulling rather than pushing, so the slack side of the belt is therefore the section between the left-hand side of the crankshaft pulley and the left-hand side of the supercharger pulley. In order to reduce the load on its bearing, a manually adjusted tensioner is usually placed on the slack side of the belt, while sprung tensioners must always be placed on this side.

If the belt is adequately tensioned, clearances to the belt can be kept quite small – even when the distance between successive pulleys is large. For example, with the 3PK belt being used here, the clearances are as tight as 8mm. The belt doesn’t foul any surfaces, even under full load and high revs.

Doing It

In this series we’ve been covering the fitting of a small Subaru Vivio supercharger to a Toyota four cylinder engine. This engine uses a single 3PK poly-vee belt to drive the air con compressor (via the ribbed side of the belt) and water pump (via the flat side). That’s all that is driven by the belt - the engine doesn’t have an external alternator and uses electric power steering.

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As shown here, the standard belt tensioner is located on an extension of the engine mount – however, to fit the supercharger, a completely new engine/supercharger mount has been constructed, so removing the tensioner. The original tensioner is of the manually adjusted type, using a grooved pulley mounted inside the belt on the slack side.

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The Subaru supercharger that’s being fitted uses a 4PK (ie 4-rib) pulley. However, given the low supercharger power draw that’s expected in this application, it was decided to stay with the 3PK belt, which will fit straight onto the 4PK pulley, leaving a groove free. (The alternative was to replace the crank pulley/damper and air con compressor pulleys with 4PK designs, something which will still be possible to do later if the 3PK belt proves to slip.)

Three drive belt approaches were considered:

  • Pivot the blower on its lower mounts to adjust belt tension. This requires a belt that is exactly 1410mm long, as the room for pivoting the supercharger is very limited.
  • Use a tensioner placed on the outside of the belt. This flat pulley could be mounted on the slack side between the crank pulley and the supercharger pulley. The placement of the idler in this position would also increase blower belt wrap. However, the available room is extremely tight and the pulley would be very hard to access to adjust belt tension.
  • Use a tensioner on the inside of the belt, placed on the tension side between the blower and the water pump. This would require a grooved pulley and would slightly decrease blower belt wrap. While space is again tight, there is more room available here than on the slack side of the belt.

It was decided that the last approach would be used.

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The bracket to hold the standard Toyota tensioner....

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...was fabricated from 40 x 8mm steel bar.

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It is mounted on a gusseted plate welded to the main supercharger bracket. The tensioner bracket bolts to the mounting plate via four high tensile Allen-key bolts.

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Slots are located in the bracket to allow the tensioner pulley to be moved sideways so that it can be precisely lined-up with the belt. The slots also allow the pulley to be twisted slightly so that its axis of rotation can be at exactly 90 degrees to the belt. (It’s important that you give your tensioner pulley some adjustments other than just for tension. In the real world, it’s unlikely that the pulley will perfectly line up without some further adjustments being available.)

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The completed bracket looks a little over-engineered but it’s important to realise that in this application it is subjected to pulsing loads imposed by the Roots-type blower. In addition to these loads, it must also withstand the loads caused by the belt tension – and do all of this without the bracket deflecting.

Another important aspect to keep in mind is that the longer the belt, the greater the tension adjustment range that will be needed. The system should be organised so that the belt is quite a tight fit to get on when the adjuster is at its lowest belt tension position.

In operation the belt drive works well, with no discernible slippage and only normal belt wear.

Laser Line-ups!

If you have a small pointer-style laser, use it to line up the pulleys when making adjustments. By shining the laser down the front faces of the pulleys, it’s easy to see small misalignments.

Next week: supercharger inlets and outlets, bypass valves, and suppressing noise

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